1. Field of the Invention
The present invention generally relates to an optical scanning device that can be employed in an image forming apparatus. The present invention specifically relates to a lens in an optical scanning device.
2. Description of the Related Art
Conventionally, optical scanning devices have been widely used in image forming apparatuses, such as optical printers, digital copiers, or optical plotters. In recent years, low-price optical scanning devices less prone to be influenced by environmental fluctuations and configured to support formation of high-definition images have been desired.
When various lenses for use in an optical scanning device are formed of a resin material, such resin-made lenses are light-weight and can be formed at low cost. Also, it is easy to form a special surface shape typified by an aspheric surface. Therefore, with a special surface being adopted for a resin-made lens, optical characteristics can be improved, and the number of lenses forming an optical system can be reduced. That is, adoption of a resin-made lens contributes to making the optical scanning device compact, light-weight, and low-cost.
On the other hand, however, the resin-made lens may be changed in shape and refractive index according to environmental changes, in particular, temperature changes. Therefore, there is a problem in which the optical characteristics, in particular, power, are changed from design values and a beam spot diameter, which is a diameter of a light spot at the time of light-gathering of light beams on a plane to be scanned, is varied according to environmental variations.
In one method, since the power variations of the resin-made lens according to temperature variations occur conversely between a plus lens and a minus lens, plus and minus resin-made lenses are included in an optical system of an optical scanning device and changes in optical characteristics occurring in these plus and minus resin-made lenses due to environmental changes are cancelled out.
Also, a semiconductor laser, which is generally used as a light source for an optical scanning device, has a property in which a light-emitting wavelength is shifted to a long wavelength side as the temperature increases (wavelength changes due to temperature changes), and also has a wavelength change due to a mode hop. A change in wavelength in the light source may cause a change in characteristics due to chromatic aberration of the optical system for use in the optical scanning device, and this change in characteristic also may cause a variation in beam spot diameter.
Therefore, in an optical scanning device including a resin-made lens in an optical system and using a semiconductor laser as a light source, optical design has to be made in consideration of changes in optical characteristics according to temperature changes and also changes in optical characteristics according wavelength changes in the light source.
One known example of an optical scanning device (laser scanning device) adopting a diffracting surface to stabilize optical characteristics in consideration of changes in optical characteristics according to changes in temperature and wavelength changes at the light source is disclosed in Japanese Patent Application Laid-Open No. 2002-287062.
Japanese Patent Application Laid-Open No. 2002-287062 discloses an optical scanning device in which laser light emitted from a laser light source is parallel light in a main scanning direction, and an optical system for light-gathering near a deflection reflecting surface of an optical deflector in a sub-scanning direction is made as “one optical element having one or more reflecting surfaces without having a rotational axis of symmetry, having two transmission surfaces provided with a power diffracting surface, and being made of resin”. Furthermore, as a comparison example, “an optical scanning device provided with a resin-made collimator lens that collimates light beams from a semiconductor laser and resin-made cylindrical lenses for light-gathering of the collimated light beams in a sub-scanning direction each provided with one diffracting surface” is disclosed. The diffracting surface is a diffracting surface having lens power due to diffraction.
Japanese Patent Application Laid-Open No. 2005-258392 discloses an optical scanning device configured to be provided with a lens having a diffracting surface before a deflector.
However, in the optical system formed of “one optical element having one or more reflecting surfaces without having a rotational axis of symmetry, having two transmission surfaces provided with a power diffracting surface, and being made of resin”, the transmission surfaces and the reflecting surfaces have to be formed in one optical element. Since a curved reflecting surface is included, manufacturing of such is not necessarily easy, and a further improvement has to be made in view of cost reduction of the optical scanning device.
Furthermore, in general, the diffracting surface requires microfabrication technology, and also requires extremely high accuracy. For example, a power diffracting surface having power equivalent to that of a spherical lens as depicted in FIG. 14A has a shape as depicted in FIG. 14B, that is, a shape obtained by folding the spherical surface so as to have a uniform height with respect to a substrate.
As evident from FIG. 14B, this power diffracting surface has narrower groove spaces as they are further distanced from an optical axis, and therefore processing these grooves is extremely difficult. Furthermore, the power diffracting surface interposed between back cuts has to form part of the spherical surface. This can be formed as a straight line for approximation, but, in this case, diffraction efficiency is inevitably decreased.
However, if the power diffracting surface is formed so as to form part of the spherical surface, roughness in surface shape is conspicuous. As a result, problems occur such that the beam spot diameter may be increased due to deterioration in wave aberration and, due to the occurrence of diffused light, ghosts may occur or light transmission efficiency may be decreased.
Still further, when a surface opposite to the diffracting surface is a refractive surface, optical characteristics are deteriorated due to eccentricity between these two surfaces.
Still further, in the optical scanning device disclosed in Japanese Patent Application Laid-Open No. 2005-258392, the optical element using a diffracting surface before the deflector is used. Due to strong power of the diffracting surface, the diffracting lens has a less tolerance to eccentricity between surfaces. Moreover, since each segment of the diffracting surface is significantly shifted from a plane, processing and measurement are difficult. As a result, excellent optical characteristics cannot be achieved.